Support for the config modifiers in the PEG notation

[andr-dots]

Arpeggio allows to set some config parameters in the initialization of the parser, but those modifiers has to be changed sometimes during the runtime. For example, in some cases you need to consume whitespace characters as regular text.

In this pull request, the config modifiers were introduced in the PEG notation, so that they can affect a single rule or a grouped expression. The config modifiers use square brackets before a parsing expression to set the config parameters. Some examples:

INDENTATION <- [whitespace='\n\r']r' *';`
SPACE <- [skip_whitespace=False]r'[ \t]+';

Also, config modifiers allow to debug erroneous rules more easily like this: [debug=True]erroneous_rules. This way, there wouldn't be a long sheet of debug output from the other rules.

This pull request depends on #131.

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